The term channel hopping is used in this document as a collective term for making hops between different information transmission channels, such as hops solely between frequencies, hops solely between time slots and hops between both frequencies and time slots in a radio communications system, for instance.
As is well known to the person skilled in this art, frequency hopping may be applied in a radio communications system to improve the performance of the radio system or as a security measure against unauthorised listening to radio communications. Frequency hopping is carried out in a predetermined order in such systems, without taking the prevailing quality of the connection into account. Frequency hopping in radio communications systems is thus not adaptive.
There can be established between a transmitter and a receiver of a radio communications system a radio connection over which radio communication can take place. The connection is two-directional, i.e. includes a downlink which forms the connection in a direction away from a base station of the system to a mobile station, and an uplink which forms the connection in the opposite direction, i.e. from the mobile station to the base station. Transmission and reception of radio traffic on different connections is effected on channels that can be defined by a given frequency in an FDMA system (Frequency Division Multiple Access), or by a combination of a given frequency and a given time slot in a system that utilizes TDMA (Time Division Multiple Access). A channel may be defined by a code in a CDMA system (Code Division Multiple Access). Seen generally, the channels that are available in a radio communications system may be disturbed significantly by other radio traffic, including radio signals on the same channels as those used for other connections, wherewith each channel in the system has a certain interference level. Thus if each connection were to use solely one channel, the connections would obtain different interference levels. The interference experienced by some connections may be so heavy as to deny an acceptable call quality. The differences in call quality of the connections can be evened-out, by jumping between different channels, wherein the connections use channels of mutually different interference levels. This spreads the use of the channels between the various connections and, with the aid of interleaving and error correction coding, more connections can obtain an acceptable call quality when the system is observed in its entirety.
Each connection may be allocated a plurality of channels, wherein the system controls the connections during ongoing communication, such as to cause the channels to hop between the channels in accordance with a given hopping rule.
This rule may, for instance, be a predetermined pseudorandom series, in which case the connections would appear to hop randomly between all available channels; cf. in this regard European Patent Application EP 93905701-4. However, this type of channel hopping may result in an unnecessarily high interference level, since the channels are not always allocated to the connections in an optimal manner when a pseudorandom series is used.
Another type of channel hopping is cyclic channel hopping. In cyclic channel hopping, a connection hops between a number of channels in accordance with a cyclically repeated channel hopping sequence.
It is well known that channel hopping can be applied in a GSM system. The GSM system is a TDMA system, meaning that each frequency is divided into a plurality of time slots that form a TDMA frame. In the GSM system a TDMA frame is comprised of eight time slots. When establishing a connection between base station and mobile station in a GSM system, the connection is allocated one of these time slots in each TDMA frame. Channel hopping is then effected, by hopping the connection between channels that have the same time slot, wherein in practice the connection hops solely between different frequencies. According to the GSM specification, a particular frequency may occur only once in a channel hopping sequence, and the frequencies in a channel hopping sequence will always occur in a rising order. On the other hand, the duration of the channel hopping sequences may vary between different base stations.
A radio communications system will normally include a plurality of channels that can be used for establishing connections between a given base station and mobile stations. It is then important that the same channel is not used simultaneously for two or more connections between the base station and mobile, stations. If two base station transmitters transmit to their respective receivers different signals simultaneously on the same channel, it is very likely that at least one receiver will be disturbed by interference deriving from the transmission to the other receiver. When this cannot occur, i.e. when only one of the base station connections can be transmitted on a channel at each point in time, it is said that the base station has orthogonality.
When a connection in a radio communications system is excessively poor, so that acceptable speech quality cannot be achieved, it may be due to an excessively low ratio between signal strength and interference. By signal strength is meant the strength of the received desired signal. By interference is meant the sum of the signal strength of all received undesirable signals on the channel used. These undesired signals arrive primarily from other connections that use the same channel in nearby cells in the radio communications system. The received undesirable signals may also arrive from those connections within the own cell that use an adjacent frequency or time slot.
The strength of the desired received signal will depend on transmitter power and on the extent to which the desired signal is attenuated in its path from the transmitter to the receiver. Attenuation is determined, among other things, by distance, direction and topology between transmitter and receiver. Other terms used in parallel with attenuation are path gain and path loss, as the skilled person is well aware.
Channel hopping in a radio communications system is described in the International Patent Application WO 96/02979. Channel hopping is effected between a plurality of channels that are each allocated a respective connection. A signal attenuation parameter, e.g. path gain, is measured in respect of the connections, which are thereafter ordered with respect to the signal attenuation parameter. The described method includes measuring a mean value of a channel quality parameter, such as interference, on individual channels. The channels are thereafter ordered with respect to the measured channel quality parameter. Only those channels that will provide the best channel quality are used for establishing the connections.
When allocating channel hopping sequences to connections, attention is paid to the connection quality of respective connections and to the channel quality of the channels in respective channel hopping sequences. A low quality connection is allocated a channel hopping sequence in which the channels used have a high quality, and a high quality connection is allocated a channel hopping sequence in which the channels used have a low (poorer) channel quality. This division of channel hopping sequences to connections ensures that orthogonality is achieved in each base station. A channel hopping sequence may include a different number of channels in different base stations. The number of channels used in a channel hopping sequence is fixed within a base station.
Swedish Patent Application SE 94022492-4 describes a method and apparatus for channel hopping in a radio communications system. A mean interference value in respect of the channels in the radio communications system is determined, or measured, for each connection. The values obtained are stored in an interference list for each of the connections involved. The values in the interference lists are then weighted and the resultant weighted lists analysed. There is then generated for each connection a hop sequence list on the basis of the analysis of the weighted lists. A channel that has a high weight value for a given connection will often occur more frequently in a corresponding hop sequence list than a channel that has a low weight value.
The aforedescribed methods provide a mean interference value. However, the interference may vary with time and consequently the channel interference may differ at different time intervals within a channel hopping sequence. It would therefore be desirable to obtain the values of the interference within those different time intervals in a channel hopping sequence in which a channel may be used.
The American patent U.S. Pat. No. 4,998,290 describes a radio communications system that utilizes frequency hopping. The system includes a central control station that allocates frequencies for communication with a plurality of participating local radio stations. The control station establishes an interference matrix that reflects the capacity requirement of the different radio stations and interference on all connections.
One drawback with this method is that it is necessary to install a central control station in the system, wherewith the system becomes more complex.
German Patent Application DE 4403483A describes a method of reorganizing frequency jump groups for an FDM/TDM radio transmission. A plurality of pre-defined frequency hop tables are stored in a base station controller, ESC. One table (TAB) is used at a time. If any connection that utilizes one of the frequency hop sequences in the table is of poor quality, the table (TAB) used on that occasion is replaced with a new table (TAB1). The switch from a current table to a new table is effected stepwise. In one exemplifying embodiment, the frequency hop sequences are changed for two time slots at a time. The switch, or change, only takes place when no information is transmitted on these time slots.
One drawback with this method is that the frequency hop tables are pre-defined. When the interference situation is not observed continuously, the best channels with regard to interference cannot be utilized in an optimal fashion. Another drawback is that attenuation of the connections is not taken into account when allocating hop sequences thereto.
The International Patent Application WO 91/13502 describes a method for carrier-divided frequency hopping. All available frequencies in the radio communications system that can be used for frequency hopping are found in a frequency pool from which channel hopping sequences are determined. Each base station is allowed to choose frequencies from the frequency pool when frequency hopping. The distance of the mobile station from the base station is taken into account when allocating a time slot to a mobile station that wishes to setup a connection. Nearby mobile stations are given the centermost time slots of a TDMA frame, whereas remote mobile stations are given time slots that lie respectively at the beginning and at the end of a TDMA frame. This is done in order to avoid overlapping of time slots (Time alignment).
The International Patent Specification WO 93/17507 describes a method of communication in a TDMA cellular mobile radio communications system that uses frequency hopping. A mobile station in one cell selects radio channels and time slots independently of a mobile station in a neighbouring cell. The hop sequences within a cell are selected so that no co-channel interference will occur. Although co-channel interference may occur between cells, it is considered that such an occurrence is only on a small scale. The power output of the mobile stations is controlled such that mobile stations that are located close to the base station will transmit on a lower power than mobile stations that are located far away from the base station.
One drawback with the methods proposed in these two latter patent specifications is that the interference situation is not taken into account when generating hop sequences. Another drawback is that attenuation of the connections is not taken when allocating hop sequences to connections.